Metal refining process



Patented Apr. 22, 1952 UNITED STATES PATENT OFFICE James B. Wagstaff,Ridgewood, NMT., assignor to Hydroca'bon Research, Inc., New York, N.Y., a cor'poration of New Jersey Application April '10, 1948, semi N.20202 l This invention relates to the refiriing of metal compositions.More particularly, it perta'ins to a process wherein oxygen isintroduced into a bath of molten crude iron to effect decarburization.

Oxygen is advantageously used in the elimination of carbon from molteniron preparatory to the manufacture of steel. A stream of thegas may beintroduced at a substantially constant rate into a bath of the metalincluding, for example, pig and scrap irons in an open hearth furnaceand the process continued until the carbon content of the metal bath isdecreased to` a predetermined amount as dictated by the type of steelwhich is to be produced. t

It has been found in practice that when the carbon content of the molteniron is decreasedby this process to below a certain figure, which,incidentally, is substantially above the desired ultimate content,increasingly greater amountspf oxygen are required to be introduced intothe i metal bath to avoid a substantial diminution in the rate of carbonelimination. This figure appears to vary with several factors includingthe compositions of the metal bath and the slag layer. stated in anotherway, after the carbon content of the iron bath has been decreased tothat certain figure (hereinafter for convenience called the criticalcarbon content), a constant rate of oxygen introduction into the moltenmetal yields a decreasingly smaller rate of decarburiz a tion or carbonelimination; `In mostcases the critical carbon content is of the orderof by weight. When the carbon content approaches this Value, anequilibrium condition is apparently created involving the iron oxide inthe slag layer and the residual carbon in the molten metal. Thisequilibrium indicates that, as the oxygen input continues after thecritical carbon content has been reached, some of the oxygen is consumedin the formation of iron oxide and thus is not available for furtherdecreasing the carbon content of the molten iron. M After the carboncontent has been brought down to the critical value by the introductionof oxygen into the molten iron, further decrease in the carbon contentof the molten iron can be effected by continuing the introduction ofoxygen at substantially the same rate over a relatively long period orby increasing the rate of oxygen introduction and thus decreasingthecarbon con: tent of the molten iron to the ultimate desired value ina shorter period. In either case, it is clear that a greater Volume ofoxygen is consumed per unit weight of carbon eliminated from the molteniron after the critical carbon content` has been passed than wasconsumed before this value was 2 ciaims. (o. 75 51) re'a'ched.Furthermore, some of the iron reacts with the oxygen to `form iron oxideafter the carbon content of the molten bath has been decreased to belowthe critical value. Thus, a portion of the iron,'which would otherwisebe recoverecl as a useful and valuable product, is undesirably converted,to iron oxide as a by-product of the refining operation.

A primary object of .the present invention is to increase the efiiciencyin the refining of crude iron by contact with oxygen in concentrateclform. i

Another important object is to accelerate the de'carburization of moltencrude iron after its carbon content h'as been decreased to the criticalvalue.

Other objects and advantages of the invention will become apparent fromthe following description considered in connection with the drawingsillustrative of the applications of the inventicn.

In accordance With the present invention, it has been found that theaforesaid disadvantages are substantially minimized by addingcontinuously or periodically, iron oxide to the metal bath after thecarbon content thereof has been lowered to the critical Value by blowingoxygen in concentrated form through the molten iron. The added ironoxide appears to block or at least materially diminish the reactionbetween the oxygen and the molten iron so that as the introduction ofoxygen is continued when the carbon content of the metal is below thecritical value a smaller proportion of the oxygen is wasted in theundesirable side reaction than is the case When iron oxide is not addedto the. bath.

The iron oxide .is preferably and conveniently introduced below thesurface of the molten metal bath by suspension in the oxygen streaminjected thereinto. Any desired iron oxide such as ferrous oxide,magnette, hematite, mill scale or the like may be used for the purposesof this invention but it ispreferred to select the more highly oxidizedforms of iron like hematite. The iron oxide is added to the bath infinely comminuted or powdered form and is rapidly dispersed under theagitation caused by the oxygen injected into the moltenbath. The addedoxide tends to collect in the slag layer.

The entrainment of the iron oxide in the oxygen stream is :thoughtadvisable since in this way the iron oxide is in the immediate presenceof the oxygen introduced into the metal bath. Also, a some- Whatimproved distributon of the iron oxide is accomplished by introducingthis agent directly into the' molten bath along with the' oxygen.

Referring to the diagrammatc drawing of Figure 1, I have shown forillustrative purposes, my invention as applied to the rening of crudeiron in an open hearth furnace. The numeral I& indicates an open hearthfurnace of well-known type having a refractory base l I on which thebath of molten metal l2 rests, this bath as usual having an upper slaglayer l'3. The top l4 of this furnace is formed with an opening |5through which extends the leading end of a metal Conduit IT, the lowerportion of which is provided with refractory sheath elements 18 forprotection of the metal pipe from the deteriorating eflect of the moltenmetal. This lance comprising pipe H and refractory elements 18 is fullydescribed and claimed in my copending application, Serial No. 3,726,filed January 22, 1948. The lower portion l of the conduit l'l dips intothe metal bath well below the slag layer !3. The oxygen is introducedinto a mixing chamber |`9 where it becomes intimately mixed withpowdered or comminuted iron oxide introduced into the mixing chamber bya screw conveyor 20 driven by a motor 21. The iron oxide is passed tothe screw conveyor from a hopper 22 through a valve 23 and thence intothe mixing chamber !9. Thus, the comminuted iron oxide is Suspended inthe oxygen stream flowing through chamber !9 and carried thereby throughthe lance into the molten metal bath. i

Figure 2 is a graphic representation of a typical heat. The carboncontent of the molten iron and the iron oxide content of the slag layerare both plotted against the time of the refining heat. As shown, thecrude iron has a high carbon content at the start of the heat, usuallyin the range of about 2 to 4% by weight. Oxygen, say about 95% pure, isinjected through the lance into the molten bath and its carbon contentdecreases because of the reaction With oxygen. The rate at which thecarbon is eliminated from the iron is fairly constant until the carboncontent of the metal falls below 1% by' weight. Depending on thecomposition of the iron bath under treatment, the rate of carbonelimination becomes appreciably smaller in many instances when thecarbon content has dropped down to about 015% and more generally whenthe carbon content has fallen to about 031% and lower. The carboncontent at which an appreciably lower rate of carbon elimination isnoted hasbeen referred to hereinbefore as the critical carbon content.If the injection of oxygen is continued at a uniform rate, the carboncontent of the iron is slowly brought down to the desired value, say0.06% by weight, in time T. Thus, under normal circumstances, the lengthof the heat period is time T. Durin this heat, it is noticed that theiron oxide content of the slag layer over the molten iron is fairlyconstant until the carbon content of the iron has fallen below about 1%by weight; in the vicinity of the critical carbon content of the iron,the slag shows an increasing content of iron oxide and this oxidecontent appears to keep rising rapidly to the end of the heat. I believethat the increase of iron oxide in the slag layer is attributable to acompetitive reaction between the molten iron and its carbon content forthe oxygenblown into the bath and that this competition for oxygenseriously afiects the rate of carbon elimination only after the carboncontent of the molten metal has been decreased to a relatively lowvalue, say 0.6 by weight. References herein to the critical carboncontent of the molten iron are more readily understood in the light ofthis theory.

In accordance with my invention, when the critical carbon content oi'the iron has been reached, say at time Te, iron oxide is introduced intothe molten bath as well as the oxygen stream. Assuming a substantiallyuniform rate of addition of iron oxide from time Te to time T2, the slaglayer shows an increasing content of iron oxide which content is madehigher (see dotted line) than that normally noted in the absence ofadded iron oxide. The added oxide, however, promotes more rapidelimination of carbon (see dotted line) so that by the end of time Tzthe carbon content of the iron is down to the value attained in theabsence of added oxide in the longer time T. stated in another way, theadded oxide appears to accelerate the rate of carbon elimination so thatif the introduction of iron oxide and oxygen were continued to time T,the carbon content of the iron would then be well below the desiredultimate content.

As graphically represented in Figure 2, my invention of adding an ironoxide to iron undergoing refining by oxygen after the carbon content hasbeen lowered to the point (critical carbon content) where the rate ofcarbon elimination is appreciably curtailed results in a shortening ofthe heat period by an amount equal to the difference between times T andT2. If the operation of my invention is carried on to time T, the carboncontent of the finished iron is lower than it would otherwise be. In anyinstance, it is evident that the addition of iron oxide to the bathafter the critical carbon content has been reached results in an economyof valuable oxygen. The added iron oxide also minimizes the amount ofiron converted to oxide during the refining treatment.

For further Clarification of the invention, a specific example is nowpresented. An open hearth furnace contains a molten bath comprising tonsof pig iron and 40 tons of scrap iron. The carbon content of the molteniron is 313% by weight at the start of the rening heat. The furnace ismaintained at a temperature of about 2900 F. Oxygen (96% by Volumepurity) is injected into the molten iron at a uniform rate of about 250cu. ft. (standard conditions) per ton of iron per hour. The injectedoxygen not only eliminates carbon from the molten iron but also oxidizesother impurities such as silicon, phosphorus and m'anganese. When thecarbon content of the molten iron has been decreased to a value of about0.4% by weight, comminuted mill scale is added to the bath at theuniform rate of 'l tons per hour while the injection of oxygen iscontinued at the aforesaid rate. The charging of the mill scale into themolten bath is preferably accomplished with apparatus of the type shownin Figure 1 wherein the mill scale is fed from hopper 22 into the mixingchamber 59 and thence conveyed by the oxygen stream through lance llinto the molten bath. The desired ultimate carbon content of the iron is`0.05% by weight and this carbon content is reached in about 1 hour fromthe time when the addition of mill scale is started. At this point therefining heat is terminated and the refined iron withdrawn from thefurnace.

In contrast to the results obtained when the refining process isconducted in accordance with the principles of this invention, thedesired ultimate carbon content of &05% by weight can be reached withoutthe addition of the mill scale used in the example or any other ironoxide if the rening period is lengthened by about 1 hour. It is obviousthat the additional hour required to reach the desired ultimate carboncontent without the addition of iron oxide will consume an additional50,000 cu. ft. (standard conditions) of oxygen for the specifiedrefining heat.

In refining crude iron by injcting concentrated oxygen into a molte nbath of the iron, I find it advisable to use oxygen of at least 80% byVolume purity and preferably of at least 90% by Volume purity. Recentdevelopments in the field of oxygen production have eventuated inprccesses which yield oxygen of about 95% by Volume purity at costspermitting the large-scale use of such oxygen in processes of the typecontemplated by this invention.

Various modications of the invention will occur to those skilled in the`art upon consideration of the foregoing disclosure without departingfrom the spirit and scope thereof. Accordingly, only such limitatonsshould be imposed as are indicated by the appended claims.

I claim:

1. The process of refinng crude iron having a oarbon content of about 2to 4% by weight to yield a rened iron having a carbon content of theorder of not more than 005% by Weight, which comprises decreasing thecarbon content of said crude iron to a carbon content of about 0.4 to 1%by weight solely by injecting concentrated oxygen of at least 80% byVolume purity into a molten bath of said crude iron, and thereafterfurther decreasing the carbon content of the iron in said molten bath toa carbon content of the order of not more than 005% by Weight bysimultaneously injecting concentrated oxygen of at least 80% by 'Volumepurity and iron oxide into said molten bath.

2. The process of refining crude iron having a carbon content of about 2to 4% by weight to yield a refined iron having a carbon'content of theorder of not more than' 005% by weight, which comprises decreasing thecarbon content of said crude iron to a carbon content of about 0.4 to06% by weight solelyby injecting concentrated oxygen of at least 80% byVolume purity into a molten bath of said crude iron, and

thereafter further decreasing the carbon content of the iron in saidmolten bath to a carbon content of the order of not more than 005% byweight by simultaneously injecting concentrated oxygen of at least byVolume purity and iron oxide into said nolten bath.

JAMES B. WAGSTAFF.

REFEREN CES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES U. S. Bureau of Mines Report ofInvestigations, No. 2502, pages 31, 32 and 33, Published in July 1923.

Basic Open Hearth Steel Making, pages 36 to 38. Published in 1944 by theA. I. M. New York, N. Y.

Open Hearth Proceedings, 1947 edition, pages 14 to 32. Published by theA. I. M. E. New York, N. Y.

The, Iron Age, Feb. 20, 1947, pages 42 to 45. Published by the ChiltonCo., Philadelphia.

Metal Progress, December 1947, pages 966 and 967. Published by theAmerican Society for Metals, Cleveland, Ohio.

Year Book of the Amer. Iron and Steel Inst. 1947, pages 234 and 262.Published by the Amer. Iron and Steel Inst., New York, N. Y.

1. THE PROCESS OF REFINING CRUDE IRON HAVING A CARBON CONTENT OF ABOUT 2TO 4% BY WEIGHT TO YIELD A REFINED IRON HAVING A CARBON CONTENT OF THEORDER OF NOT MORE THAN 0.05% BY WEIGHT, WHICH COMPRISES DECREASING THECARBON CONTENT OF SAID CRUDE IRON TO A CARBON CONTENT OF ABOUT 0.4 TO 1%BY WEIGHT SOLELY BY INKECTING CONCENTRATED OXYGEN OF AT LEAST 80% BYVOLUME